Most telephone calls are completed without the aid of an attendant, and the calling customer is informed of the disposition of the call by tone signals or voice announcements transmitted over the connection. In order to provide good service, it is desirable for the telephone company to be apprised of any difficulties that may be encountered in the completion of calls automatically and without an attendant's assistance.
In the past, calls were monitored by bridging a service-observing attendant to the connection to observe the progress and disposition of the call. Thus, the observer could record the number dialed by the calling party and then monitor the transmission path for audible ringing, busy, recorder, and supervisory signals which indicated the disposition of the call. The observer would then make a record of the call and discontinue monitoring before any conversation began.
In the interest of maintaining privacy in the telephone system and automating the service-observing function, arrangements were devised for monitoring a telephone connection and automatically ascertaining the nature of the tones returned to the calling customer.
Typical signals present on telephone lines which indicate the disposition of the call are audible ringing, busy and reorder. The audible ringing signal is heard at the calling station when the called station is being rung to inform the calling customer that the connection has been completed. This tone continues until the called customer answers or the call is abandoned. Busy and reorder signals, on the other hand, are heard when the called line is busy or when no circuits or equipment are available, such as during traffic overloads or equipment failures.
Generally, the signals comprise voice frequency tones that are combined linearly or by modulation wherein the combined tones are then interrupted at a preassigned rate or cadence. For example, the audible ringing signal may comprise a series of two-second tone bursts wherein adjacent bursts are separated by a four-second silent interval. The busy and reorder signals generally comprise a combination of tones interrupted at 60 and 120 times per second, respectively.
Many of the automatic prior art service-observing arrangements ascertain the nature of these signals primarily through the recognition of their cadence. While these arrangements are suitable for their intended purpose, they require that the signal be present for a sufficient length of time so that a repetitive pattern can be recognized. Also, since these frequencies fall within the voice frequency range, they are highly susceptible to interference from the calling party who may inadvertently introduce signals which frustrate the recognition of the cadence.
Certain prior art service-observing arrangements rely on bandwidth filters which respond only to those frequencies normally used for audible ringing, busy and reorder signals. While these arrangements are wholly suitable for their intended purpose, it must be recognized that precise filtering arrangements cannot be used with any degree of reliability. The reason for this is that audible ringing, busy and reorder signals are generated by sources at many different switching offices, and the frequencies may vary from one office to the next. Furthermore, the methods in which the signals are generated differ throughout the switching network. For example, to generate an audible ringing signal, one office may linearly combine 440 Hz and 480 Hz tones while another office may modulate a 420 Hz tone with a 40 Hz tone. Likewise, busy and reorder signals may be generated by two methods, namely, the linear combination of 620 Hz and 480 Hz tones or by a method wherein a 600 Hz tone is modulated by a 120 Hz or 133 Hz signal. At any given switching office, however, the busy and reorder signals are usually generated by the same method and differ only in cadence.
While the modulation of linear combination methods of generating signals have different frequency components, the signals nevertheless, provide the listener with comparable sounds having similar "buzz" or "pitch" rates. For example, in the case of modulated audible ringing signals, the 40 Hz envelope or pitch rate of the signal may be heard by the customer, while with the linearly combined signals the station transducers and human ear perceive a similar pitch rate sensation between 30 Hz and 50 Hz for the listener even though no energy is present on the line at the 30 Hz to 50 Hz rate.
Similarly, busy and reorder signals have a characteristic pitch rate within the 90 Hz to 150 Hz range that is discernible to the calling customer whether the signal was generated by the linear combination method or the modulation method outlined above.
Thus, measuring the energy level precisely at one frequency, say 420 Hz for an audible ringing signal, may be suitable for detecting the modulated version of an audible ringing signal, but this would be wholly unreliable for detecting an audible ringing signal which comprises the linear combination of 440 and 480 Hz.